The present invention relates to ultrasound transducers and, in particular, it concerns cylindrical ultrasound receivers and transceivers formed from piezoelectric films, and their applications in digitizer systems.
It is known to employ cylindrical ultrasound transducers for transmitting ultrasound signals in digitizer systems. The cylindrical form provides all-around signal transmission and simplifies the geometry of time-of-flight calculations by providing an effect similar to a point (or more accurately, line) source. These advantages are detailed in U.S. Pat. No. 4,758,691 to De Bruyne. A further advantage of cynlindrical ultrasound transducers is that they can be centered around an element of which the position is to be measured. This is used in a drawing implement digitizer system described in PCT publication WO98/40838.
Structurally, a number of different types of cylindrical transducer have been proposed. the De Bruyne patent proposes a xe2x80x9cSell transducerxe2x80x9d which is capacitive device formed from a complicated arrangement of cylindrical layers intended to produce a cylindrical air gap of about 20 xcexcm. Such a structure is costly to manufacture, and is likely to be unreliable.
A second type of transducer which has been proposed in the field of medical applications is based on piezoelectric elements. An example of a medical transducer of this type may be found in U.S. Pat. No. 4,706,681 to Breyer et al. which discloses an ultrasonic marker. Here, a cylindrical piezoelectric collar is sandwiched between two electrodes. Application of an alternating potenial across the electrodes causes vibration of the collar, and hence emits a radially propagating ultrasonic signal.
In principle, any ultrasonic transducer is capable of being operated both as a transmitter and a receiver. In practice, however, many considerations result in many transmitter structures being ineffective as receivers. This is particurlarly true of cylindrical elements in which almost the entire cylinder contributes to wide angle transmission by actuation with a relatively high power while only a small portion of the cylinder is correctly oriented for receiving an incoming signal from a given direction. Furthermore, the inherent capacitance of the large inactive region of the transducer may absorb a large proportion of the amplitude of a received signal, rendering the transducer insensitive as a receiver.
In the field of transducers in general, much work has been invested in development of devices based on piezoelectric films, such as PVDF. Conductive electrodes are formed on opposite faces of the film, typically by selectively printing conductive ink on regions of the surfaces. These films are cheap to produce, and withstand a wide range of operating conditions including exposure to moisture.
Although a cylindrical ultrasound transducer is relatively simple to implement using piezoelectric film, implementation of a receiver poses additional problems beyond the general complications of cylindrical receivers discussed above. Specifically, referring to FIGS. 1 and 2, there is shown a schematic plan view of a freely suspended cylinder 10 formed from piezoelectric film. FIG. 1 shows its relaxed state, while FIG. 2 shows the response of cylinder 10 to an incoming ultrasound signal wave front 12. Since the piezoeletric film is flexible, the oscillations of signal 12 generate waves (exaggerated for clarity) traveling around cylinder 10. The direction and extent of flexing of the piezoelectric film varies along the wave form created around the cylinder, resulting in reversal of the sense of an electrical potential generated between the electrodes. As a result, much of the potential generated by the piezoelectric film may be dissipated in local eddy currents within the electrodes, greatly reducing the overall signal voltage as measured between the electrodes.
A further problem of implementing a cylindrical ultrasound transducer using pieoelectric film is the tendency for the electrode to act as an antenna picking up unwanted electromagnetic radiation which may result in very low signal to noise ratios.
There is therefore a need for a cylindrical ultrasound receiver structure employing pieoelectric film.
The present invention is a cylindrical ultrasound receiver structure employing pieoelectric film.
According to the teachings of the present invetnion there is provided, an ultrasound receiver comprising: (a) a hollow cylinder formed primarily from flexible pieoelectric film, the hollow cylinder having an outer surface, an inner surface, a central axis and a height measured parallel to the central axis; (b) a sensing electrode formed from conductive material applied to the inner surface; (c) a grounded electrode formed from conductive material applied to the outer surface; and (d) a support structure for supporting the hollow cylinder, the support structure being configured to support the hollow cylinder in such a manner as to allow propagation of vibration waves circumferentially around a major part of the hollow cylinder, wherein the sensing electrode is formed as a strip extending in an extensional direction substantially parallel to the central axis along a major part of the height, the strip subtending at the central axis an angle of not more than 90xc2x0.
According to a further feature of the present invention, the strips subtends at the central axis an angle of not more than 30xc2x0.
According to a further feature of the present invention, the grounded electrode extends over a major part of the outer surface.
According to a further feature of the present invention, there is also provided at least one additional electrode formed conductive material applied to the inner surface in pattern non-contiguous with the sensing electrode.
According to a further feature of the present invention, the at least one additional electrode extends over a major part of the inner surface.
According to a further feature of the present invention, the at least one additional electrode is grounded.
According to a further feature of the present invention, configured for use additionally as an ultrasound transmitter, thereby serving as an ultrasound transceiver, the ultrasound transceiver further comprises a control module including: (a) receiver circuitry electrically connected to the sensing electrode; (b) transmitter circuitry; and (c) a switching system associated with an actuating electrode selected from the grounded electrode and the additional electrode and configured to alternately electrically connect the actuating electrode to the transmitter circuitry and to ground.
According to a further feature of the present invention, the support structure includes a conductive core element deployed within the hollow cylinder in such a manner as to avoid electrical contact with the sensing electrode, the conductive core element being electrically grounded. According to one preferred implementation, the conductive core element is a metal core element. According to an alternative omplementation, the conductive core element is formed from conductive foam.
According to a further feature of the present invention, the flexible piezoelectric film is implemented as PVDF film. p According to a further feature of the present invention, the sensing electrode and the grounded electrode are implemented as substantially transparent electrodes.
There is also provided according to the teachings of the present invention, a method for operating an ultrasound transceiver for receiving and transmitting ultrasound signals, the method comprising the steps of: (a) providing an ultrasound transceiver structure including: (i) a hollow cylinder formed primarily from flexible piezoelectric film, the hollow cylinder having an outer surface, an inner surface, a central axis and a height measured parallel to the central axis, the hollow cylinder being mounted so as to allow propagation of vibration waves circumferentially around a major part of the hollow cylinder, (ii) a sensing electrode formed from conductive material applied to the inner surface, the sensing electrode being formed as a narrow strip extending in an extensional direction substantially parallel to the central axis along a major part of the height, the strip subtending at the axis an angle of not more than 90xc2x0, (iii) at least one additional inner electrode formed from conductive material applied so as to extend over a major part of the inner surface in a pattern non-contiguous with the sensing electrode, and (iv) at least one outer electrode formed from conductive material applied so as to extend over a major part of the outer surface; (b) receiving ultrasound signals by: (i) connecting both the additional inner electrode and the outer electrode to ground, and (ii) electrically connecting the sensing electrode to receiver circuitry; and (c) transmitting ultrasound signals by applying a driving voltage to at least one of the additional inner electrode and the outer electrode.
There is also provided according to the teachings of the present invention, a method for operating a system for determining the position of a movable element, the system including a first group of movable ultrasound transducer including al least one ultrasound trandsducer associated with a movable element and a second group of fixed ultrasound transducers including at least two ultrasound transducers maintained in fixed geometrical relation by attachment to a base unit, the method for operating including: (a) operating the system in a measurement mode in which: (i) one of the first and second groups of ultrasound tranducers trnasmits at least one measurement signal which is received by ultrasound transducers in the other of the first and second groups, and (ii) a position of the movable element is derived from time-of-flight measurements for the at least one measurement signal; and (b) intermittently operating the system in a calibration mode in which: (i) at least one ultrasound transducer from the second group transmits a calibration signal and at least one other ultrasound transducer from the second group receives the calibration signal, and (ii) calibration information is derived from time-of-flight measurements for calibration signal.
According to a further feature of the present invention, at least one ultrasound transducer from the second group is implemented as the aforementioned cylindrical ultrasound transducer structure.
There is also provided according to the teachings of the present invention, a method for providing mechanical protection for an ultrasound transducer used for a given frequency of ultrasound signals while minimizing interference with the ultrasound signals, the method comprising positioning a protective grating adjacent to the transducer, the grating having plurality of openings spaced at a spatial frequency of less than about half, and preferably less than about a quarter, of the wavelength of the given frequency of ultrasound in air. For a cylindrical transducer, the grating is preferably configured as a cylindrical grating surrounding the transducer.